WHEN I WAS YOUNG, I spent hours in the yard just outside my house. I loved mixing soil and water, then apply to or mix with sticks, stones and the like. By the end of playtime things tended to get even muddier winding up on my face and clothes.
As I trained as an artist, I still wallowed in "mud" of the painting kind, but not by choice. Try as I might, I couldn't create the colors I wanted...only mud. Tube colors were the only guaranteed way be free from problems, but pure tube colors were "not allowed" by most instructors.
Today, people ask me how I keep my colors so clean, clear, and free from mud. The answer is, now I am "mud conscious". I can make clear colors when I want to, and mud if I want to. It's called control. Even neutral or broken colors can glow.
Here are a few tips for avoiding "mud". These are not in any particular order. Experiment with those which apply to your situation:
If you are painting on a white surface, use a white palette. If on a neutral surface, use a neutral colored palette, etc. The better you can emulate the color of the painting surface with your palette, the closer color match you'll achieve from the start (if using pastel or colored pencil: place a piece of like paper beside your painting to test on before touching your painting.)
Whatever light you have directed on your painting, duplicate that light on your palette. If painting in a studio with a florescent light on your painting, have another light of the same type on your palette/mixing area. If you work en plein air (in the open air) have your easel and paint in the same lighting condition. Don't have your palette in the sun and your painting in the shade (generally, you want to be in a diffused light not direct when working outside). It will take extra effort to make corrections. Many students use varying light sources which can contribute to distorted colors.
Try to isolate the color using a small piece of cardboard with a hole in it viewing with one eye, or simply curl your fingers to create a viewer. Look, look, then look some more, but don't stare. Switch your eyes back and forth relating the color to it's neighbors to more accurately evaluate what it is. What do you see? What does the color seem to have most in of, red, green, blue, yellow, violet, orange? Select the basic tube color that comes closest to what you see (in the case of pastels or colored pencils, choose the closest color stick to what you see in the subject). What else can you see in the color? Is it a greenish red, or perhaps a bluish green, etc.? Add a SMALL quantity of the second color you think will adjust the first resulting in the color you need.
Or try it this way; first match the actual color, then it's intensity, then the temperature, then adjust for value. For instance, to warm a red without changing it's intensity, try adding another warm color such as a deep yellow first. If the red is too intense, add it's equal green compliment in a small quantity to break the intensity without lowering the value.
Raising or lowering the value without changing the intensity is easy if you use another color before resorting to white or black. White is wonderful, but it can a major contributor to mud if not used wisely. To lighten without changing the intensity, add a lighter, analogous color. Resort to white after trying lighter colors first. To darken a color, the same principle applies. Add the next darkest color, or, if you wish to gray down yet maintain brilliance, use the compliment a little at a time. To achieve a glowing warm or cool "black" without using black, experiment with mixes of the darkest, richest complimentaries first, such as viridian & alizarin, or ultramarine & burnt umber.
Limit your palette. You've probably heard this many times before, but it can help you learn how to manage color. Then add colors as you gain confidence and familiarity.
Test the colors you already have. Every tube in a color family and between brands is different. Not all blues and reds will combine to make nice violets. In fact, some are mud makers! Make color wheels and charts using the colors you currently use. See which combine into mud and which do not. Becoming familiar with the benefits and limitations of your paint can go a long way to assuring more consistent, desirable results.
Read Susan Sarback's book, Capturing Radiant Color in Oils, by Northlight Books (beneficial to those using any media, not just oil paint).
This book will assist in identifying how you currently use color then help you "see" and methodically capture light in your paintings as never before.
Unfortunately, I read this book after I discovered how to avoid mud, but it really reinforced and gave new insights on how to maximize my efforts.
Capturing Radiant Color in Oils
If you are running into trouble while painting and things are looking muddy, don't add more color. Chances are you have a paint buildup or are overworking an area. Remove some of the paint and begin again. If using oil, you can wait till it dries a bit then reapply paint; if acrylic, let it dry and reapply or use glazes of color over top to make adjustments; if watercolor, try lifting some color out first, and for pastel/colored pencil, wash the surface, use a kneaded eraser, lightly spray with fixative, or gently remove with an exacto knife then reapply color.
Achieving beautiful, clear color is a matter of educated yet intuitive trial and error. Make notes while mixing so you can duplicate colors that are working and avoid those that are not. Estimate what you think will work then slowly and carefully mix. At this point let me encourage you to read, study, and take classes about color with professional artists...and of course, with the WetCanvas! team.
If there is one thought I can leave with you about avoiding muddy colors it is this...mix only what you need to create the desired color, starting with two colors first. Only add a third if you really need it. If you are careful and selective you'll usually end up with what you want. I am not advocating timidity however; be brave. As I frequently tell my students, "You are the boss of the colors. Take charge, and don't let them get the best of you." And when in doubt about mixing, I just pretend I'm a child playing in my yard again having fun!